For example, in Japanese Patent Laid-Open No. 2005-163678, in order to rotate a rotary impeller, which is rotatably supported, through a control of an external magnetic field, a rotor magnet is buried in the inside of the rotary impeller by insert molding. When insert molding is utilized, a rotor magnet which is inserted can be buried at a predetermined position accurately (positional accuracy is higher) and thus rotation of the rotary impeller can be controlled accurately.
The technique may be applied to a rotary impeller which is used, for example, to measure a flow rate of fluid flowing through a predetermined space. In other words, a magnet (magnetic body) which is paired with a magnetic detecting element for sensing magnetism is buried in the rotary impeller by insert molding to improve a measurement accuracy of a rotation number of the rotary impeller, i.e., a flow measurement accuracy of fluid.
However, when the rotary impeller in which a magnetic detecting member such as a magnet is buried is to be molded by insert molding described in the above-mentioned Patent Reference, an insert pin for supporting (positioning) the magnet and the like is required to advance and retreat to and from a die cavity and thus the die is complicated.
Further, in a case that insert molding is utilized, a hole left in a molded product after an insert pin for supporting the magnetic detecting member such as a magnet has been retreated from the cavity is usually buried by thermal fusion or potting in order to prevent the magnetic detecting member from being exposed to the outside. However, it is difficult to completely seal the hole having been left as a trace of the insert pin by thermal fusion or potting. Therefore, in a case of a rotary impeller which is used for flow measurement of food such as an automatic ice making device or a packaging process of a beverage, fluid may enter into the inside where the magnetic detecting member is buried and, when the magnetic detecting member is a magnet, the magnet may be corroded and it is hygienically undesirable. Further, when thermal fusion or potting is used, a gap space may be formed between the resin used to bury an opening and the magnetic detecting member and, in this case, the magnetic detecting member is rattled in the inside of the impeller. In a product in which rattling occurs, as described above, a distance between the magnetic detecting element and the magnetic body which affects detection accuracy of rotation for each product is not stabilized and thus detection accuracy of rotation is lowered.